Aluminum containers are typically manufactured by a drawing and forming operation known as draw-ironing or drawing and ironing. This process results in the deposition of lubricant and forming oil on the container surface. In addition, small fragments of leftover aluminum are often deposited on the surface and are present in relatively large quantities on the interior surface of the container. The container surface is cleaned prior to, for example, conversion treatment or painting of the container, and the surface must be free of contaminants which would result in less than an excellent water wettability and thus impair subsequent container processing.
At present, the compositions normally employed commercially to clean aluminum containers are aqueous sulfuric acid solutions containing hydrofluoric acid and at least one surfactant or aqueous solutions containing phosphoric acid, nitric acid, or Fe.sup.3+, and sulfuric acid and at least one surfactant. These cleaning solutions are extremely effective and offer many advantages, but they nevertheless suffer from certain types of problems inherent to such acidic cleaning compositions. Thus, for example, these compositions can dissolve and corrode the stainless steel equipment or other ferrous alloy equipment which is typically used for a container cleaning line. Moreover, discharge of any hydrofluoric acid and fluoride present in the rinse water and spent cleaning bath causes environmental problems. In the case of Fe.sup.3+ -containing cleaning solutions, the iron hydroxide present in the preliminary hot water rinse prior to the cleaning step may stick in the heat exchanger.
Alkaline cleaning solutions have already been formulated in an attempt to solve these problems; however, known alkaline cleaning solutions are themselves associated with problems which impair their commercial application. For example, when the use of an alkali metal hydroxide containing cleaning solution is attempted, an irregular etch is often obtained with broad range of aluminum containers. Also, when the line is interrupted due to operational problems downstream from the container cleaning line while the spray alone continues to operate, black smut is produced from aluminum alloy components due to excessive etching. Such containers are commercially useless. In addition, the hydroxide layer continues to grow on the surface of the aluminum after an alkaline cleaning and becomes substantially thicker than the hydroxide layer after an acidic cleaning. A thick hydroxide layer creates problems in any subsequent conversion treatment and accordingly results in a poor corrosion resistance. Finally, magnesium is segregated to the aluminum surface after an alkaline cleaning of alloys that include magnesium, and this causes, inter alia, an unsatisfactory paint adherence.
Thus, in order to remove this hydroxide layer and segregated Mg, it becomes necessary to implement an acidic wash, for example by nitric acid, after an alkaline cleaning. However, plant space and available equipment considerations make it difficult to introduce an acid wash step into the container cleaning line. In sum, prior alkaline cleaning solutions exhibit various problems as detailed above.